Provide HAVE_XXXX_H defines for C headers (take 3)

==================================================
PR#2093, PR#2361, PR#2377, PR#2434,
PR#2524, PR#2525, PR#2533, PR#2569

Background
----------
Currently we have some places (especially in conf.h and os/unix/os.h) where we
make decisions about the existence of particular header files based on the
underlying platform. In general this is sufficient, but expercience showed
(especially the Linux/dlfcn.h PRs) that making general implications from
platforms to file existance is very problematic because of too much variants
and version of particular platforms.

Problem
-------
The problem is that we decide particular header files exists even when they
don't exist. This caused compile time failures and a lot of PRs.

Solution
--------
The short-hand solution in the past was to use ``-D<PLATFORM>=<VERSION>''
defines instead of ``-D<PLATFORM>'' and use ``#if defined(<PLATFORM>) &&
<PLATFORM> >= <VERSION>'' instead of ``#ifdef <PLATFORM>''. This is ok for
platforms where we really have access to and where we can be sure that a
particular platform version has a file or hasn't it.  This is not the case for
esoteric Unix derivates and not the case of the huge amount of Linux variants.

So a long-term solution is needed. As often GNU Autoconf is the father of the
idea: We actually check for the existance of particular header files and
define HAVE_XXXX_H if ``#include <xxxx.h>'' works (which means xxxx.h exists).

The patch
---------
The patch actually does the following:

1. It src/Configure another check-cycle is done where we test for various
   header files which later conditionally can be included via

     #ifdef HAVE_XXX_H
     #include <xxx.h>
     #endif

   This is done by calling TestCompile with a new command "header" which
   checks for existing header files.  As a result the file include/conf_auto.h
   is generated containing these HAVE_XXX_H defines.  Currently the following
   header files are tested (can be extended in the future):

     dlfcn.h
     dl.h
     bstring.h
     crypt.h
     unistd.h
     sys/resource.h
     sys/select.h
     sys/processor.h

2. In include/conf.h all HAVE_XXX_H defines/undefines were replaced by
   a single ``#include "conf_auto.h"'' statement at the top. And in all
   related files the inclusion of such headers are now based on HAVE_XXX_H
   defines.

The generated conf_auto.h file looks like this:

  | /* Automatically generated file - DO NOT EDIT */
  | #ifndef APACHE_CONF_AUTO_H
  | #define APACHE_CONF_AUTO_H 1
  |
  | /* <dlfcn.h> */
  | #ifndef HAVE_DLFCN_H
  | #define HAVE_DLFCN_H 1
  | #endif
  |
  | /* <dl.h> */
  | #ifdef HAVE_DL_H
  | #undef HAVE_DL_H
  | #endif
  |   :
  |   :
  | #endif /* APACHE_CONF_AUTO_H */

Submitted by: Ralf S. Engelschall
Reviewed by: Dean Gaudet, Jim Jagielski
PR: 2093, 2361, 2377, 2434, 2524, 2525, 2533, 2569


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81704 13f79535-47bb-0310-9956-ffa450edef68